New Cars Leave MPG Standard Behind - WSJ

General Motors promises its forthcoming Volt hybrid electric car will push fuel-economy levels to new heights. It also could spark an overhaul of the miles-per-gallon standard, a number that doesn't tell consumers enough about the next generation of vehicles.

In recent weeks, GM has touted the Chevrolet Volt's expected 230 mpg fuel economy in city driving. The big number dwarfs the mileage of any car on dealer lots.

Yet high mileage claims for the Volt and other planned plug-in automobiles highlight a deep flaw with the mpg standard: As automobiles increasingly rely on multiple fuel sources, or on electricity alone, gauging their efficiency in terms of gasoline risks giving consumers inaccurate information about the financial and environmental costs of driving.
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* MPG Fits Awkwardly in Electric-Vehicle Landscape

One problem is that in hybrid vehicles, mileage variation could be extreme, depending on which fuel source is being used.

In addition, the Environmental Protection Agency hasn't finalized rules for how it will measure fuel economy on the Volt or other cars that can be plugged into an electrical outlet. Until then, ...